1. Field of the Invention
The present invention relates to a semiconductor light emitting device, in more particular, to a high luminance type semiconductor light emitting device.
2. Related Art
As to a light emitting diode (LED) which is a semiconductor light emitting device, it is possible to fabricate a high luminance LED emitting various color lights such as blue, green, orange, yellow, and red, since it is possible to grow a GaN based high quality crystal or an AlGaInP based high quality crystal by using MOVPE (Metal Organic Vapor Phase Epitaxy) method in recent years. In accordance with the provision of a high luminance LED, applications of the LED are widened, e.g. a back light of a liquid crystal display, a brake lump for a vehicle so that demand for the LED increases year by year.
Since the growth of a high quality crystal by using the MOVPE method has been achieved, a light emitting efficiency inside the light emitting device is nearing to a theoretical limit value. However, the light extract efficiency from the light emitting device to the outside is still low, and enhancement of the light extract efficiency is important.
For example, a high luminance red LED is made from AlGaInP based materials, and has a double hetero structure comprising a conductive GaAs substrate, an n-type AlGaInP layer comprising an AlGaInP based material with a composition which is lattice-matched with the conductive GaAs substrate, a p-type AlGaInP layer, and an active layer which is a part of a light emitting part comprising AlGaInP or GaInP, in which the active layer is sandwiched by the n-type AlGaInP layer and the p-type AlGaInP layer.
Since a bandgap of the GaAs substrate is narrower than that of the active layer in such a semiconductor light emitting device, most of the light emitted from the active layer is absorbed by the GaAs substrate, so that the light extract efficiency is significantly deteriorated.
As means for solving this problem, there is a technique for improving the light extract efficiency by forming a layer with a multilayer reflective film structure comprising semiconductor layers having different refractive indices between the active layer and the GaAs substrate, to reflect the light emitted to the GaAs substrate, thereby reducing absorption of the light in the GaAs substrate. However, according to this technique, only the light having a limited incident angle with respect to the multilayer reflective film structure layer is reflected.
Thus, Japanese Patent Laid-Open No. 2002-217450 discloses another technique for realizing a high luminance by forming a semiconductor light emitting device in which a double hetero structure part comprising AlGaInP based material is grown on a GaAs substrate for growth, sticking the double hetero structure part on a Si supporting substrate via a metal layer with a high reflectance, and removing the GaAs substrate used for the growth. According to this technique, since the metal film is used as a reflective film, the reflection with high reflectance can be realized without selecting an incident angle with respect to the reflective film.
In addition, the reflective film comprising a metal having a high reflectance such Au, Al, and Ag cannot be joined by ohmic-contact with an AlGaInP based semiconductor. Accordingly, it is necessary to provide an ohmic contact portion comprising a material different from a metal composing the reflective film at an interface between an AlGaInP based compound semiconductor layer and the reflective metal film, to reduce an electrical resistivity.
In general, the ohmic contact portion comprises a Zn based metal, since a contacting surface is a p-type semiconductor layer.
However, in case of using the Zn based material, a forward voltage cannot be reduced enough. This tendency is remarkable, when an area of an ohmic contact portion in the LED bare chip is small.
As a technique for reducing the forward voltage, there is a technique of elevating a heat treatment temperature. However, according to this technique, an alloying reaction occurs between the semiconductor layer and the ohmic contact portion, so that a light absorption rate of the semiconductor layer contacting with the ohmic contact portion is increased. In addition, the light absorption in a reflective metal film other than the ohmic contact portion is increased, since a reaction between the reflective metal film and other metals is promoted. In other words, there is a disadvantage in that deterioration of a light output is involved when the heat treatment temperature is elevated to reduce the forward voltage.
Accordingly, it is necessary to use a material for the ohmic contact portion by which the contact is easily provided compared with the Zn based material. A Be based material is one of such materials.
However, according to conventional semiconductor light emitting device using the Be based material for the ohmic contact portion, there is a disadvantage in that the light output is low and a reliability of the device is low, although the forward voltage can be reduced compared with the semiconductor light emitting device using the Zn based material for the ohmic contact portion.
In other words, it is difficult to reduce the forward voltage while keeping the light output. This tendency is remarkable, when the area of the ohmic contact portion in the LED bare chip is small. This problem is a big issue for providing a higher output of the semiconductor light emitting device in future.